The invention relates to a plasma treatment device which contains two microwave plasma sources coupled to one another, and to a method for operating a plasma treatment device of this kind. In particular the plasma treatment device is a continuous feed system, through which a substrate to be treated is passed by means of a transport device during the plasma treatment.
Plasma treatment devices are used for the processing of substrate surfaces, in particular for the deposition or removal of layers on or from the substrate surface and for the modification of the properties of the substrate surface. Here, the particles (molecules, atoms or electrically charged particles) necessary for the processing are excited or produced with the aid of a plasma. For large substrates, continuous feed systems or inline systems are often used, in which a plurality of plasma sources can also be arranged in succession in the transport direction of the substrates, as appropriate.
For the generation and maintenance of the plasma, there are various possibilities for introducing the energy necessary for this purpose into a gas or gas mixture. One possibility is excitation by means of microwaves (frequency in the range of 900 MHz to 10 GHz), whereby high plasma densities can be achieved in the generated plasma, and therefore high deposition rates or high etching rates based on a chemical reaction. Microwave plasma sources are described by way of example in DE 198 12 558 A1 and DE 103 41 239 A1. The plasmas excited using microwaves of this kind form plasma boundary layers with low boundary layer potentials as the plasmas interact with walls or substrate surfaces. The charge carriers (ions and electrons) contacting the substrate surface thus only have a low energy, usually less than 10 eV. However, for improved control of the processing, for example for the control of layer properties, such as density, strength or stress of a deposited layer, greater and/or controllable energies especially for the ion currents to the substrate surface are desirable.
For this purpose, the microwave plasma can be superimposed by an electric field, one electrode of which is provided by the substrate to be processed or a substrate carrier, on which the substrate is supported during the processing. To this end, a defined reference potential is applied to the substrate or the substrate carrier, which potential causes an acceleration of the electrically charged particles contacting the substrate surface, in particular ions, from the plasma to the substrate surface in the electric field. In stationary systems, the substrate carrier can usually be directly contacted. For continuous feed systems, a capacitive coupling of the substrate carrier to a defined potential, for example ground, is known, for example from DE 10 2012 103 425 A1.
However, this purely capacitive coupling of the substrate carrier to a defined potential and therefore the generation of a defined electric field is unstable and can be controlled only inadequately. In addition, parasitic plasmas may form, and these are undesirable.